Detachment Folding in the Kurdish Zagros: Hidden Structures and Missed Opportunities

Abstract

In the last several years, the Zagros belt along the NE edge of the Arabian plate generated much interest due to new opportunities in the Iraqi part of the belt. Despite often great outcrop conditions and well exposed anticlines in the High Fold belt, at least 10 wildcat wells in Iraqi Kurdistan missed the crest and drilled down steep flanks instead. In the Low Fold belt, blocks picked up before seismic data was available turned out not to contain the expected major thrusts and associated traps, but only a large syncline and no trap at all. Many, if not most, of these companies had extensive experience in exploring fold-thrust-belts, so what happened? The main reason is the structural style. Most anticlines in the High Fold belt are poorly imaged seismically because surface exposure is dominated by carbonates. The lack of seismic image calls for model-based interpretation. Yet the usual fault-bend fold, fault-propagation fold and trishear fold models don't seem to apply and with poor seismic data it is difficult to plan a well guaranteed to hit the crest. Numerous internal detachments dominate the style of deformation in the Mesozoic and Cenozoic. This led to a specific style of fold belt, which is not dominated by thrusts and stacked sheets, but by detachments, complex folds, and only the occasional thrust ramp. Most anticlines are steep-limbed detachment folds, with multiple internal detachments, locally overturned limbs, wedge-thrusts, and generally minor breakthrough thrusts that lead to many surprising geometries that are difficult to predict. One of the major issues is forelimb-thickening by secondary folding between minor detachments, particularly in the lowermost Jurassic. Crests of deeper horizons are thus located farther hinterward, sometimes significantly (> 1km). Additionally, anticlines with two steep limbs may have a very tight core area, or reservoir horizons may be completely cut out of the visible anticline. This type of deformation has been observed at many other locations world-wide. Detachment folds are a rather common occurrence and outcrops and image logs through steep limbs of folds often show a similar style of secondary folding in many other places. The smaller scale of these folds tends to make this folding style less relevant for the industry. In the Zagros, however, numerous detachment horizons and rheological contrasts led to a style of deformation that has surprised even experienced operators.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016